The present invention is related to an automatic gain control circuit suitably used to control a gain of an audio appliance.
In general, automatic gain control circuits (will be referred to as “AGC circuits” hereinafter) have been provided at a pre-stage of a power amplifier in output stage of an audio appliance such as speaker reproducing apparatus and recording apparatus. The above-described AGC circuit is a circuit equipped with a function capable of controlling a gain when input audio signals are amplified in order that even under such a condition that peak levels of waveforms of the input audio signals are varied, the input audio signals are amplified in proper levels and then the amplified input audio signals having the proper levels are supplied to a power amplifier. There are some possibilities that this sort of AGC circuits are employed as means for preventing occurrences of clips in power amplifiers employed in output stages of audio apparatuses. It should be understood that these AGC circuits are disclosed in, for instance, JP-A-9-93063, and JP-A-2000-106511. Further, the clip preventing technique using the AGC circuit is disclosed in JP-A-2000-106511.
On the other hand, the clip preventing techniques utilizing the conventional AGC circuits are described in, for example, JP-A-2000-106511 as the conventional technical idea. That is, in JP-A-2000-106511, temporal averaged values of output signal voltages of a power amplifier are detected; and a gain of an amplifier provided at a prestige of the power amplifier is fed back in a negative feedback control manner in such a manner that the detected averaged value becomes constant in order that peak levels of output signal waveforms of the power amplifier may be converged to such a level lower than, or equal to a clip level. Although such utilization modes of these AGC circuits have merits to prevent clips, there is such a problem that since non-linear elements such as VCA and the like are used, the output signals of the power amplifiers are distorted.
Further, in the AGC circuits, it is required to set an attack time and a release time to an appropriate length according to the application of the AGC circuits. The attack time is a time for lowering a gain of the amplifier in response to a level of the input audio signal when the level increases, and the release time is a time for increasing the gain of the amplifier in response to the level of the input audio signal when the level is decreased. In the related art, in the case that the AGC circuit is implemented by a semiconductor integrated circuit, a time constant circuit having a time constant in response to the attack time and the release time is externally provided to the AGC circuit which is the semiconductor integrated circuit so that the AGC circuit performs an operation for obtaining the attack time and the release time. Therefore, number of parts of the whole audio appliance including the AGC circuit increased, and thereby increasing the cost. Further, the time constant of the time constant circuit may be sensitive to the temperature and may change over time. Thus, if this circuit is used or the AGC circuit, the problem arises in which stabilities of the attack time and the release are poor.